To understand the energy balance over the complex terrain of the Loess Plateau, we analyzed data from the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL). From 1 June to 15 August 2008, the energy balance closure ratio ranged from 0.52 to 0.90 during the daytime when the available energy was greater than 150 Wm(-2); during nocturnal hours, it was about 0.25. We proposed a method based on water mass conservation to estimate the energy transmitted by the vertical movement of moisture in the soil and set out to explain the imbalance. During the daytime, the heat stored in the soil above the heat flux plate placed at 5 cm below the surface, the energy transmitted by the vertical movement of moisture in the soil, and the energy assimilated by plant photosynthesis contributed to 29.6%, 2.0%, and 1.8% of the energy residual, respectively. During stable conditions, the soil heat storage contributed to 46.4% of the energy residual, whereas the other energy terms contributed little. The instantaneous energy closure ratio was about 0.80 during unstable conditions; it was about 0.40 when the wind speed U was greater than 3.0 ms(-1), while depends systematically on U under condition of U<3.0 m s(-1) during stable conditions. Under the weak wind stable condition, affected by topography-induced nonstationary motions, the turbulence is anisotropic with a strong horizontal fluctuation and a weak vertical fluctuation, resulting in weakened heat mixing in the vertical direction and stronger unclosure of energy.